The representation of space is critical for the coordination of many forms of human perceptual and cognitive behavior. The overarching question addressed in this project is, what psychological and neural mechanisms underlie the process of spatial representation? While there is general acceptance in the animal and human literature that spatial information is neurally instantiated in the parietal lobe, there is little agreement on the frame of reference within which spatial representations are defined. Indeed, findings from single cell studies in animals, psychology experiments with normal subjects and neuropsychology investigations with patients all yield somewhat contradictory results. Three major issues are addressed in the current work: (a) whether spatial information is represented with respect to coordinate frames defined by the viewer and/or coordinates independent of the viewer; (b) what factors (task demands, stimulus characteristics and strategies) influence the strength of a particular representation, and (c) whether a 'context' spatial frame of reference serves to coordinate information across perception and action. The primary methodology involves the detailed analysis of the behavior of neurological patients who, following a lesion to the parietal lobe, have a selective deficit in spatial behavior (neglect). Through evaluating the pattern of their impaired and intact performance on a range of tasks that require access to internal spatial representations, we can obtain evidence for the mechanisms underlying spatial behavior. The first set of experiments compare the performance of the patients on tasks requiring eye movements, visually guided reaching and visual target detection. In these experiments, the contribution of individual spatial frames of reference are partialled out to permit an analysis of their unique contribution to behavior. The second set of experiments compare performance on tasks which manipulate variables thought to be crucial to spatial representation, including symmetry, principal axes, and motion. The final experiments evaluate the preferential reliance on a particular representation through manipulating stimulus expectancies and strategies. The results of these studies will provide evidence for two central issues. The first concerns the way in which spatial information is normally represented and organized. The ease with which sensory information is located and coordinated across modalities for action belies the complexity of the process of spatial representation. Findings from these studies promise to provide important insights into the mechanisms underlying this process. The second issue concerns the function of parietal cortex and its breakdown in patients with cortical lesions. An improved understanding of what gives rise to neglect might lead to better diagnostic and remediation procedures for this fairly common neurobehavioral disorder.